In the field of an image forming apparatus which uses an electrophotographic image forming method, image forming apparatuses of a so-called intermediary transfer type, which form a full-color toner image on an intermediary transfer belt (ITB), are known. Among high speed image forming apparatuses of this type, some are enabled to keep their endless belt within a preset range in terms of the lengthwise direction of one of rollers by which the endless belt is suspended and kept tensioned (direction parallel to one of rollers), by detecting an amount of positional deviation of the endless belt such as the intermediary transfer belt, conveyance belt, and the like, and controlling the roller in alignment.
One of the primary problems which image forming apparatuses such as those described above suffer is color deviation attributable to the positional deviation of their endless belt in terms of a primary scanning direction, and a secondary scanning direction (parallel to belt conveyance direction), of a laser scanner, stretching or shrinking of toner image in terms of the primary scanning direction, angular deviation of toner image relative to the primary scanning direction, and the like. Another problem is that the apparatuses change in toner density due to inaccuracy in the components related to development, transfer, and the like, changes in ambient temperature and humidity, cumulative usage of apparatus, and the like factors, and therefore, the apparatuses become nonuniform in terms of image density.
Thus, some image forming apparatuses of the above-described type are structured to form a pattern to be used for compensating for color deviation density deviation, as means for measuring the amount of color deviation, on the intermediary transfer belt with preset intervals, detect the pattern with the use of a sensor, and correct the apparatuses in image formation position and image density. In the case of these apparatuses, in order to accurately detect the amount of color deviation and image density, the inaccuracy in the positional relationship between the endless belt and sensor has to be minimized.
In the past, there has been disclosed in Japanese Patent No. 3473346, for example, an apparatus structured so that a sensor of a reflection type is attached to a frame of a belt unit, and the frame is positioned relative to a shaft of an idler roller which is one of rollers by which an intermediary transfer belt is suspended and kept tensioned, with the placement of a positioning plate between the frame and the shaft (rotational axle) of the idler roller. According to this art, the belt unit is equipped with an endless belt such as an intermediary transfer belt, for example, and a preset detection mark (test patch) is formed on the endless belt by an image forming section. Then, the information which can be obtained by detecting the test patch with the use of an optical sensor is used.
According to the conventional arts which include the one described in Japanese Patent No. 3473346, the distance between the endless belt and the sensor of the reflection type can be kept stable with the use of the positioning plate. However, with the use of only the conventional arts, it is difficult to keep the sensor stable in its angle relative to the surface of the intermediary transfer belt, for the following reason. That is, the angle of the belt unit in terms of the direction indicated by a referential mark θ (angle of belt unit at plane perpendicular to shaft (rotational axle) of belt-suspending-tensioning roller) is determined by a main assembly of an image forming apparatus, whereas the angle of the belt surface is determined by the belt-suspending-tensioning roller of the belt unit. On the other hand, the angle of the optical sensor relative to the belt surface in terms of the direction indicated by the arrow mark θ is determined by the positioning plate attached to the main assembly of the image forming apparatus.
That is, the angle of the sensor of the reflection type relative to the belt surface is determined by a combination of the angle of the belt surface relative to the belt unit, and the angle between the positioning plate and sensor, including their deviation. Therefore, with the use of any of the conventional arts, it has been difficult to ensure that the angular deviation of the optical sensor relative to the belt surface in terms of the direction indicated by the referential mark θ remains minimum.